1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same. In particular, the present invention relates to an optical scanning device that is suitably used for an apparatus such as a laser beam printer or a digital copying machine having an electrophotographic process, in which a light flux optically modulated and emitted from light source means is reflected and deflected on a polygon mirror serving as optical deflection means and then a surface to be scanned is scanned with the light flux through a scanning optical system to record image information. The present invention relates to an optical scanning device capable of providing a satisfactory image by adopting a diffraction grid for correction of chromatic aberration of magnification or temperature compensation and to an image forming apparatus using the same. In addition, the present invention relates to a color image forming apparatus which uses a plurality of optical scanning devices and is composed of a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
Up to now, in an optical scanning device used for a laser beam printer (LBP) or the like, a light flux optically modulated according to an image signal and emitted from light source means is periodically deflected by, for example, an optical deflector composed of a rotating polygonal mirror (polygon mirror). The deflected light flux is converged to form a spot shape on a photosensitive recording medium (photosensitive drum) by a scanning optical system having an fθ characteristic. The surface of the recording medium is scanned with the light flux to perform image recording.
FIG. 13 is a schematic view showing a main part of a conventional optical scanning device.
In FIG. 13, a divergent light flux emitted from a light source means 1 is converted into a substantially parallel light flux by a collimator lens 3. The substantially parallel light flux is limited by a diaphragm 2 and incident into a cylindrical lens 4 having predetermined refractive power only in the sub scanning direction. Of the substantially parallel light fluxes incident into the cylindrical lens 4, the light flux within a main scanning section outgoes therefrom without changing an optical state. The light flux within a sub scanning section is converged and imaged as a substantial linear image onto a deflection surface (reflection surface) 5a of a deflecting means 5 composed of a polygon mirror.
The light flux which is deflected on the deflection surface 5a of the deflecting means 5 is guided onto a photosensitive drum surface 8 serving as a surface to be scanned through a scanning optical system 6 having an fθ characteristic. By rotating the deflecting means 5 in a direction indicated by an arrow “A”, the photosensitive drum surface 8 is scanned with the light flux in a direction indicated by an arrow “B” to record image information.
Further, in order to achieve high speed scanning, a multi-beam optical scanning device that simultaneously forms a plurality of scanning lines by light fluxes from a plurality of light sources has been proposed and commercially available from various companies. FIG. 14 is a schematic view showing a main part of a multi-beam optical scanning device. Two light fluxes emitted from light sources 81 and 82 are converted into parallel light fluxes by collimator lenses 83 and 84 and then synthesized into one by a synthesizing optical element 85. The synthesized light flux forms a linear image extended in the main scanning direction near a deflection surface of a polygon mirror 87 by the action of a cylindrical lens 86 and then forms a light spot on a photosensitive drum 89 by a scanning optical system 88. Therefore, the two scanning lines can be formed by performing optical scanning once, so that extremely high speed scanning can be achieved as compared with a conventional optical scanning device. With respect to a multi-beam light source other than one using the above-mentioned synthesizing optical element, a monolithic multi-beam laser in which a large number of light emitting points exist has been produced. In the case where the monolithic multi-beam laser is used, it is unnecessary to use the synthesizing optical element. Thus, it is possible to simplify the optical system and the optical adjustment.
In an optical scanning device using a multi-beam light source, in order to eliminate a jitter caused by a wavelength difference between a plurality of light sources (variation in interval between scanning lines on the photosensitive drum surface in the main scanning direction), any countermeasure such as the appropriate selection of the light sources has been taken so as to minimize the wavelength difference between the light sources. When the jitter caused by the wavelength difference between the light sources (chromatic aberration of magnification) is corrected by the scanning optical system, a plurality of lenses having different dispersion characteristics are required. As compared with a scanning optical system that does not correct the chromatic aberration of magnification, the number of lenses generally increases to cause an increase in cost. There is a limitation with respect to the range of selection of the wavelengths of the light sources. Therefore, it is difficult that the wavelengths are made completely equal to one another. There is a problem with respect to a cost for the selection of the wavelengths. When a semiconductor laser is activated, an image quality is deteriorated by a variation in wavelength, which is called mode hopping. Thus, even in an optical scanning device other than the optical scanning device using the multi-beam light source, in order to improve the stability of the image quality, it is necessary to minimize the jitter caused by the variation in wavelength.
A semiconductor laser used as a conventional light source (as disclosed in, for example, Japanese Patent Application Laid-Open No. H10-197820 and Japanese Patent Application Laid-Open No. H10-068903) is an infrared laser (780 nm) or a visible laser (675 nm). However, in order to realize a high resolution, the development of an optical scanning device in which a minute spot shape is obtained by using a short wavelength laser having an oscillating wavelength of 500 nm or less is under way. The advantage of the use of the short wavelength laser is that a minute spot size which is about half of a conventional spot size can be achieved while an exit F number of the scanning optical system is kept equal to a conventional one. In the case where a spot size is reduced to half of the conventional spot size while using the infrared laser, it is necessary to increase the intensity of the scanning optical system to an intensity about two times larger than that in a conventional case. A focal depth is proportional to a wavelength of a used light source and to the square of the exit F number of the scanning optical system. Therefore, to obtain the same spot size, the focal depth in the infrared laser becomes equal to or smaller than about ½ of the focal depth in the short wavelength laser.
In such an optical scanning device, in order to record image information with high precision, it is necessary to preferably correct a curvature of field over the entire surface to be scanned, to have a distortion characteristic (fθ characteristic) with a constant speed, between an angle of view θ and an image height Y, and to make spot sizes on the image plane uniform at respective image heights. Various optical scanning devices or various scanning optical systems that satisfy the optical characteristics like those have been proposed up to now.
According to Japanese Patent Application Laid-Open No. H10-197820 and Japanese Patent Application Laid-Open No. H10-068903 as described above, a (temperature compensation) optical scanning device using a diffraction optical element for a scanning optical system has been proposed to reduce a focal variation on a surface to be scanned due to the correction of the chromatic aberration of magnification and an environmental variation.
In particular, in an optical scanning device using a short wavelength light source having a wavelength of 500 nm or less, a dispersion characteristic of a material used for a scanning lens is large. Therefore, the chromatic aberration of magnification becomes six times or seven times larger than that in a conventional infrared laser. Thus, in the optical scanning device using the multi-beam laser, the jitter is significantly caused in the main scanning direction to reduce the image quality.
When a blue-violet laser which is made of a material such as gallium nitride and oscillated at the wavelength of 405 nm is used to obtain a spot size which is about half of a spot size of the infrared laser, as described above, the focal depth is proportional to the wavelength. Therefore, only a depth about half the conventional depth can be allowed. Thus, along with the improvement of precision of respective parts composing the optical scanning device, a (temperature compensation) optical scanning device in which the focal variation is not caused even in the case of the environmental variation is desired.
As described below, a phase function φ for determining a grid shape is inversely proportional to the wavelength. Therefore, when a diffraction grid having the same power is designed, a grid pitch of the short wavelength laser having 500 nm or less becomes smaller than that of the conventional infrared laser.
For the above-mentioned reason, when a light flux having a short wavelength of 500 nm or less is imaged on the surface to be scanned in the optical scanning device using the short wavelength light source having a wavelength of 500 nm or less, there is a problem in that a grid size becomes smaller.
In general, as shown in FIG. 15A, a grid shape formed on a mold by mold forming is transferred to an optical resin member or an optical glass member. After that, as shown in FIG. 15B, the transferred grid shape is separated from the mold to produce the diffraction optical element. Alternatively, a method of dropping a small amount of ultraviolet curable resin on a glass substrate (lens can be also used) serving as a base and curing the grid shape similarly formed on the mold by ultraviolet light is used.
However, as the grid size becomes smaller, the following problems are caused:
it is hard to produce the mold by a cutting tool;
transfer property of the grid shape formed on the mold deteriorates; and
along with the deterioration of the transfer property, an imaging performance deteriorates and a diffraction efficiency reduces.